Umm, but what I really wanted to say was MacOSX!
DH who is a PC guy says XP over Vista. He refuses to buy a machine with Vista pre-installed.
I'm a network engineer and we only use and recommend XP. Vista isn't that bad, it's just bloated and really takes away from the functionality.
If you won't get a Mac, wait for Windows 7, it will arrive in early fall.
I dislike both operating systems, but I would pick XP if I had to choose between the two. I have Vista on my laptop which makes it incredibly slow, despite having at least twice the amount of recommended power. We have XP on our home computer, which runs faster than my laptop, even though it is much older and has less RAM, a slower processor, etc.